Messages in this thread | | | Subject | Re: [PATCH v3 3/3] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state | From | Julien Grall <> | Date | Thu, 25 Apr 2019 16:57:26 +0100 |
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Hi Dave,
On 24/04/2019 14:17, Dave Martin wrote: > On Tue, Apr 23, 2019 at 02:57:19PM +0100, Julien Grall wrote: >> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c >> index 5313aa257be6..6168d06bbd20 100644 >> --- a/arch/arm64/kernel/fpsimd.c >> +++ b/arch/arm64/kernel/fpsimd.c >> @@ -92,7 +92,8 @@ >> * To prevent this from racing with the manipulation of the task's FPSIMD state >> * from task context and thereby corrupting the state, it is necessary to >> * protect any manipulation of a task's fpsimd_state or TIF_FOREIGN_FPSTATE >> - * flag with local_bh_disable() unless softirqs are already masked. >> + * flag with {, __}get_cpu_fpsimd_context(). This will still allow softirqs to >> + * run but prevent them to use FPSIMD. >> * >> * For a certain task, the sequence may look something like this: >> * - the task gets scheduled in; if both the task's fpsimd_cpu field >> @@ -155,6 +156,56 @@ extern void __percpu *efi_sve_state; >> >> #endif /* ! CONFIG_ARM64_SVE */ >> >> +DEFINE_PER_CPU(bool, fpsimd_context_busy); >> +EXPORT_PER_CPU_SYMBOL(fpsimd_context_busy); >> + >> +static void __get_cpu_fpsimd_context(void) >> +{ >> + bool busy = __this_cpu_xchg(fpsimd_context_busy, true); >> + >> + WARN_ON(busy); >> +} >> + >> +/* >> + * Claim ownership of the CPU FPSIMD context for use by the calling context. >> + * >> + * The caller may freely modify FPSIMD context until *put_cpu_fpsimd_context() >> + * is called. > > Nit: it may be better to say "freely manipulate the FPSIMD context > metadata". > > get_cpu_fpsimd_context() isn't enough to allow the FPSIMD regs to be > safely trashed, because they may still contain live data (or an up to > date copy) for some task.
Good point, I will update the comment.
> > (For that you also need fpsimd_save_and_flush_cpu_state(), or just use > kernel_neon_begin() instead.) > > [...] > >> @@ -922,6 +971,8 @@ void fpsimd_thread_switch(struct task_struct *next) >> if (!system_supports_fpsimd()) >> return; >> >> + __get_cpu_fpsimd_context(); >> + >> /* Save unsaved fpsimd state, if any: */ >> fpsimd_save(); >> >> @@ -936,6 +987,8 @@ void fpsimd_thread_switch(struct task_struct *next) >> >> update_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE, >> wrong_task || wrong_cpu); >> + >> + __put_cpu_fpsimd_context(); > > There should be a note in the commit message explaining why these are > here. > > Are they actually needed, other than to keep > WARN_ON(have_cpu_fpsimd_context()) happy elsewhere?
It depends on how fpsimd_thread_switch() is called. I will answer more below.
> > Does PREEMPT_RT allow non-threaded softirqs to execute while we're in > this code?
This has nothing to do with PREEMPT_RT. Softirqs might be executed after handling interrupt (see irq_exit()).
A call to preempt_disable() will not be enough to prevent softirqs, you actually need to either mask interrupts or have BH disabled.
fpsimd_thread_switch() seems to be only called from the context switch code. AFAICT, interrupt will be masked. Therefore, holding the FPSIMD CPU is not necessary. However...
> > > OTOH, if the overall effect on performance remains positive, we can > probably argue that these operations make the code more self-describing > and help guard against mistakes during future maintanence, even if > they're not strictly needed today.
.... I think it would help guard against mistakes. The more I haven't seen any performance impact in the benchmark.
[...]
>> -/* >> - * Save the FPSIMD state to memory and invalidate cpu view. >> - * This function must be called with softirqs (and preemption) disabled. >> - */ >> +/* Save the FPSIMD state to memory and invalidate cpu view. */ >> void fpsimd_save_and_flush_cpu_state(void) >> { >> + get_cpu_fpsimd_context(); >> fpsimd_save(); >> fpsimd_flush_cpu_state(); >> + put_cpu_fpsimd_context(); >> } > > Again, are these added just to keep WARN_ON()s happy?
!preemptible() is not sufficient to prevent softirq running. You also need to have either interrupt masked or BH disabled.
> > Now I look at the diff, I think after all that > > WARN_ON(preemptible()); > __get_cpu_fpsimd_context(); > > ... > > __put_cpu_fpsimd_context(); > > is preferable. The purpose of this function is to free up the FPSIMD > regs for use by the kernel, so it makes no sense to call it with > preemption enabled: the regs could spontaneously become live again due > to a context switch. So we shouldn't encourage misuse by making the > function "safe" to call with preemption enabled.
Ok, I will switch back to the underscore version and add a WARN_ON(...).
> > [...] > > Also, have you tested this patch with CONFIG_KERNEL_MODE_NEON=n?
AFAICT, CONFIG_KERNEL_MODE_NEON has always turned on by default on arm64.
I will have a look took hack Kconfig and see if it is still build.
Cheers,
-- Julien Grall
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